1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium which comprises a substrate and a multi-layered magnetic film composed of ferromagnetic metal layers and non-magnetic metal layers which are laminated alternately on top of the other.
2. Description of Related Arts
The recent advance in information society demands data recording apparatus, such as hard disk drive (xe2x80x9cHDDxe2x80x9d hereinafter), with higher performance. Improving the performance of HDD is synonymous with increasing the recording density. One promising technology to meet this demand is the perpendicular magnetic recording system which magnetizes the medium perpendicularly to the film plane, rather than in the plane. There are several candidates as the magnetic material for the recording layer of the perpendicular magnetic recording medium. Among them are CoCr-based alloys incorporated with Pt, Ta, B, etc. as additives, which have been investigated most closely.
A recording medium of CoCr-based alloy is superior in recording characteristics, but CoCr-based alloy itself has insufficient perpendicular magnetic anisotropy energy (Ku) which holds the magnetization in its orientation. For this reason, CoCr-based alloy gives an M-H curve with a squareness ratio much smaller than 1 and permits small reversed magnetic domains to appear in magnetic recording domains, thereby reducing the strength of reproduced signals. Even though its M-H curve has a squareness ratio close to 1, CoCr-based alloy is still susceptible to xe2x80x9cthermal signal lossxe2x80x9d which is a phenomenon that the state of magnetization changes after recording due to thermal disturbance.
To cope with this situation, there has been proposed a recording medium in which the recording layer is a multi-layered film composed of thin films (with a thickness of an atomic order) laminated alternately on top of the other. This recording layer can be designed such that the magnetic structure formed by the magnetic head remains unchanged with time owing to its sufficiently large value of Ku.
The structure of the multi-layered film mentioned above is generally referred to as superlattice structure. It takes on a variety of physical properties owing to the state peculiar to the interface between atomic layers. It is known that the magnetic thin film of multi-layered structure has a large perpendicular magnetic anisotropy energy if it is formed from ferromagnetic metal (such as Co and Fe) and noble metal (such as Pd, Pt, and Au) laminated alternately. This multi-layered structure of ferromagnetic metal and noble metal is used in the perpendicular magnetic recording medium disclosed in U.S. Pat. Nos. 4,587,176, 4,678,721, and 5,106,703.
Unfortunately, the magnetic recording medium with the magnetic thin film mentioned above has a larger recording noise than the perpendicular recording medium of CoCr-based alloy. In other words, it is not necessarily superior in recording properties. The cause of recording noise arises from the reversal mechanism of the multi-layered magnetic film as has been pointed out in many studies.
The fact that CoCr-based alloy thin film is capable of low-noise recording is because there exist ferromagnetic fine particles in the film, with each particle being magnetically isolated by a phase with rich Cr which has spread out on its periphery. By contrast, the multi-layered magnetic film, which takes no special effort to form the fine structure in the magnetic film, permits magnetization reversal to take place in a large area as a unit. Therefore, the magnetic domain for recording has a zigzag contour regardless of the distribution of the magnetic field applied by the recording head. This zigzag contour reflects the random distribution of the magnetic properties of the magnetic film, and hence it brings about recording noise. To address the problem involved with the multi-layered magnetic film, comprehensive studies have been made on a variety of additives and underlying materials.
Attempts have been made to use a multi-layered magnetic film of Co and Pd or Pt as a magnetic recording medium. Its composition, structure, and manufacturing method intended for better recording characteristics are disclosed in Toku-Hyo-Hei 11-501755 (Japanese translation of PCT international publication WO96/24927) According to this disclosure, each magnetic metal layer is made of Co or Co alloy with a thickness of 0.15-1.0 nm and a noble metal layer with a thickness of 0.5-1.5 nm. The number of the magnetic metal layers, each including one noble metal layers, is 10-30. The magnetic recording medium of this structure has a coercive force larger than 2.5 kOe. In addition, according to the disclosure, the multi-layered magnetic film and its nuclei-forming layer (such as Pd) should have a total thickness smaller than 150 nm so as to avoid an unnecessarily large space between the recording head and the backing soft magnetic layer (NiFe).
Moreover, an effective way of reducing the noise of recording medium is also disclosed in the patent publication just mentioned above. The disclosure mentions that the recording film should be made by sputtering with an oxygen-containing sputtering gas at a reduced degree of vacuum and at a high sputtering gas pressure, and annealing should be performed before and after the film forming step. The disclosure further mentions that it is possible to reduce the recording noise if the Co alloy layer is formed from CoCr or CoCrTa.
Another perpendicular magnetic recording medium with a multi-layered magnetic film of Pd/CoCr is disclosed in Appl. Phys. Lett. 64 (21) pp. 2891-2893 by B. M. Lairson et. al. This thesis deals with how the recording characteristics vary depending on the thickness of the CoCr layer and the amount of Cr added. With the thickness of the Pd layer fixed at 0,4 nm, it concludes that the recording characteristics are satisfactory if the content of Cr is about 15 at %.
The present inventors repeated the procedure mentioned in the foregoing patent publication (11-501755) to reproduce the sample No. 12 given in page 22. This sample is a multi-layered magnetic recording film composed of ferromagnetic layers of CoCr-based alloy and noble metal layers which are laminated alternately on top of the other. The recording film is of superlattice structure consisting of CoCr layers (0.35 nm) and Pd layers (1.0 nm), with a Pd underlying layer (20 nm). The CoCr-based alloy contains 12 at % Cr. The reproduced sample was actually tested for recording and reproducing performance. The test results indicate that the sample is stable to thermal disturbance but is not particularly superior to the conventional CoCrPt recording medium (proposed by Takano et al., Digest of Intermag 2000, AD-06).
The recording medium reproduced by the present inventors has a higher level of recording noise than the CoCrPt medium (just mentioned above) despite incorporation with Cr. A probable reason for this is that magnetic exchange interaction between particles is not sufficiently reduced. On the other hand, the magnetic film has a greatly decreased level of saturation magnetization (xcx9c100 kA/m) because the CoCr-based alloy contains 12 at % Cr. This results in a significant decrease in reproduced signal intensity. It was found that the S/N ratio of the reproduced recording medium is lower by 10 dB or less than that of the conventional CoCrPt recording medium, when measured with the same recording/reproducing head due to the increased noise (N) and the decreased signal (S).
The reason why the reproduced recording signal shows the sufficiently low recording noise is that Cr in the ferromagnetic layer of CoCr-based alloy does not fully block magnetic exchange interaction between magnetic fine particles. The laminate structure disclosed in the above-mentioned patent publication (11-501755) is characterized in that the ferromagnetic layer of CoCr-based alloy (of 0.35 nm thick) is relatively thinner than the noble metal (Pd) layer (of 1.0 nm thick). This thickness ratio is inadequate to reduce the magnetic exchange interaction between magnetic particles to such an extent as to suppress recording noise because there exists the magnetic exchange interaction between particles also in the Pd alloy layer due to magnetization induced in the Pd alloy layer by the ferromagnetic layer of CoCr-based alloy. In addition, the small saturation magnetization is apparently due to the excessively thin ferromagnetic layer of CoCr-based alloy.
However, the above-mentioned report by Lairson et al. shows (in FIG. 5) how the perpendicular magnetic anisotropy energy (Ku) depends on the thickness of the CoCr-based alloy layer. It is shown that the Ku value decreases with the increasing thickness of CoCr layer to such an extent that it is not useful for the magnetic recording medium. Moreover, in the above-mentioned report, the Ku value is merely a little over 1xc3x97105 J/m3 (FIG. 4) in the case of a multi-layered magnetic film composed of Pd layers (0.4 nm thick) and CoCr layers (0.2 nm thick) containing 15 at % Cr, which gave comparatively good results in recording and reproducing experiments. The reason for this small Ku is the insufficient surface magnetic anisotropy energy (Ks) The foregoing suggests that the magnetic layer for recording cannot be applied in the practical perpendicular magnetic recording system simply because its axis of magnetization is perpendicular to the magnetic recording medium. Tackling this problem motivated the present invention. Accordingly, it is an object of the present invention to provide a perpendicular magnetic recording medium characterized by reduced recording noise such that the magnetic particles become less susceptible thermal disturbance.
In order to achieve the above-mentioned object, the present inventors studied the magnetic recording medium having a multi-layered magnetic film composed of ferromagnetic metal layers containing Co and non-magnetic metal layers containing Pd which are laminated alternately on top of the other. The result suggests that it is possible to obtain a perpendicular magnetic recording medium exhibiting sufficient perpendicular magnetic anisotropy energy with the recording noise reduced below a certain level, if the multi-layered magnetic film has a specific composition and layer structure and is produced under specific conditions.
The present invention is based on this finding. Thus, the gist of the present invention resides in a perpendicular magnetic recording medium having a substrate and a multi-layered magnetic film formed thereon with or without an underlying layer interposed between them. The multi-layered magnetic film is composed of ferromagnetic metal layers containing Co and non-magnetic metal layers containing Pd which are laminated alternately on top of the other, characterized in that the ratio of film thickness defined by d1/d2 ranges from 1.5 to 4.0, where d1 denotes the thickness of each of said ferromagnetic metal layers and d2 denotes the thickness of each of said non-magnetic metal layers which is between 0.6 nm and 2.0 nm (0.6 nm  less than d2  less than 2.0 nm).
The perpendicular magnetic recording medium constructed as mentioned above may be modified such that the ferromagnetic metal layers are formed from CoCr-based alloy. The resulting multi-layered magnetic film has a low level of recording noise due to the reduction in magnetic exchange interaction between magnetic particles and also has good resistance to thermal disturbance due to the sufficient perpendicular magnetic anisotropy energy to resist the demagnetizing field energy induced by magnetization of the magnetic film.
The gist of the present invention resides also in a process for producing the perpendicular magnetic recording medium, which comprises a step of forming a first underlying layer on a substrate which may contains, ex. Pd, a step of forming a second underlying layer which may contains, ex. CoCr40, then a multi-layered magnetic film composed of alternately laminated ferromagnetic metal layers containing Co and non-magnetic metal layers containing Pd.
The above-mentioned process is preferably modified such that an initial layer containing a paramagnetic Co alloy interposed between the underlying layer and the multi-layered magnetic film. In addition, the step of forming the multi-layered magnetic film is preferably followed by heat treatment in a vacuum at a temperature higher than 350xc2x0 C.